JeffOYB

I see MTBs often doing very well in CX races.

Are CX bikes actually underbiking a lot of courses and going slower than if a bike had really wide tires?

Maybe a MTB in a "real" CX race is held to the same 33mm tire limit?

Seems like a modern high end 26" (or 29"?) tire might be handmade, or tubular, and have supple sidewalls, be really light and roll darn fast despite being heavier than a CX tubie. ...Absorb a lot more bumpage. ...26" might be quite nimble and, if a 2.1" tire is allowed, would get better traction in corners. ?

Where does a supposedly lighter CX bike with skinnier tires have an advantage over a 26" MTB on a CX course?

I'm noticing racers on MTBs finishing really high up, or winning, even on gravel grinders and courses with a fair amount of smooth hardpack and pavement. (Dunno if they're width-limited or not, tho.)

Velocivixen

Higher bottom bracket.

Also, when you fall off a 29er you've got farther to go before landing in the mud vs 26".

FrozenK

On a course with significant sections of hard pack or pavement, the cross bike will have an advantage. On a course with singletrack a MTB will have an advantage. On deep mud skinny tires often have an advantage. If you have long run ups a cross bike is usually an advantage due to lower weight.

Part of the fun for me, though, is to use a cross bike. Otherwise it becomes a short-track mountainbike race. Which can be fun, but it isn't a cyclocross race.

As for using a mountainbike on a "real" CX race, if you mean UCI sanctioned then flatbars and suspension are not allowed.

grolby

If people are regularly crushing your local races on MTBs, the courses are probably poorly designed. I can't think of a cross race I've done this season where I would have been faster on a mountain bike, and maybe only one or two where I would have been almost or about as fast.

grolby

Whoops, missed this. Common misconception, here: suspension is not forbidden by the UCI. Yes, flat bars are against the rules, you must use drops. But suspension is completely legal, and many athletes including Sven Nys are racing suspended cyclocross bikes (the Trek Boone) at high-level races.

FrozenK

You are correct, it isn't explicitly banned but I think it counts on the "no innovations allowed unless specifically approved" rule.

caloso

My observation is that there are Expert (now Cat 3?) and above mountain bike racers killing the C and B races with superior bike handling skills.

grolby

It might be subject to approval, but again, the Trek Boone is being raced in international competition right now. So it seems as though this approval has not been a problem so far. That rule is strange and byzantine and I don't think it's intended to cover something like a suspension system. I believe it exists to allow the UCI to insta-ban something if they decide they don't want it in competition, rather than because they seriously expect every little change to a bike to be presented to them for approval. I don't think it renders suspension illegal at all.

FrozenK

In my opinion, and based on the UCI treatment of things like disc brakes, active suspension (i.e. a suspension fork) is not allowed by UCI for cyclocross racing. The Boone is different since it relies on passive suspension (frame flex), not much different than the Niner RDO post.

And the UCI has always looked at things as "banned unless specifically allowed" rather than "allowed unless specifically banned." But anyway, a mountainbike would not be allowed on a CX race because of flat bars to start with. And once you throw drop bars on it, the main advantage you could have is tire size, which is another no-go with tires capped at 33mm.

Andy_K 

Former Team Kona rider Erik Tonkin owns a shop that sponsors a lot of local races, and he races a lot...sometimes on a MTB. I think he decides based on the course whether or not to use a MTB. Most of the time he's on a cross bike. Either way he finishes near the top. I've seen him win a race on a mountain bike and wearing a T-shirt and denim shorts. That is to say, a fast rider will be fast no matter what bike he/she is on. A cross bike versus a mountain bike is a relatively smaller differentiator. Some people are just more comfortable on a mountain bike.

I've tried racing on a mountain bike exactly once. I took both the MTB and the CX bike out during pre-ride and for that course the MTB felt better. Then the consistency of the mud changed before my race and on the first lap my 26x2.3" tires took on so much mud I could barely carry the thing. Luckily I had the CX bike in the pit.

Wide tires are great for traction, but they are usually a lot heavier than CX tires. Spinning up a heavier tire once doesn't take much more energy. Doing it over and over for 45 minutes can take a toll.

I've toyed with the idea of putting 700c wheels with CX tires on my 26" mountain bike just to get the suspension for a couple of courses where I think that might help. The biggest thing stopping me from trying it is I like sticking to the traditions of the sport. When you're slow like I am things like tradition matter a lot more than gaining a few seconds here or there.

grolby

The distinction you draw between "active" and "passive" suspension isn't a real one. But it doesn't matter. We straight up don't know if a cross bike with traditional suspension would be banned because as far as I know, no one has tried it before. Except on the road, of course, where precedence suggests they would allow it. I disagree about the UCI applying this standard as "everything is banned unless specifically allowed." That's not the history. Yes, the UCI often applies the rules capriciously, but the wording of that rule is vague as to forbid absolutely anything new, however innocuous. Yet, teams still enter races with new technology. Disc brakes, as far as I recall, were specifically banned in cyclocross, not banned under that rule, but I don't really know for sure.

But anyway, on CX vs MTB more generally: on a cyclocross course, cyclocross bikes are almost always faster. If local races are being won on mountain bikes, that's a red flag either for field quality or course design. The narrow tires on cross bikes are often cited as a major disadvantage and that's true to an extent, but I think the extra weight and inertia of bigger tires makes them a liability not far above the UCI limit of 33mm. Because the best available equipment is restrained by UCI rules even though most racers aren't subject to those rules, it's annoyingly difficult to test that theory, but oh well.

FrozenK

I'm pretty sure discs were not banned, just not allowed, because the rule was modified to explicitly allow discs for cyclocross racing. Like the UCI explicitly says that BMX and mountainbike have their own set of equipment rules.

On the rest of it, I agree. I think flat bars can be an advantage on turns, and on some circumstances a mountainbike's better traction could help. But overall, on a cyclocross course a cyclocross bike should be faster.

jfmckenna

Pretty much everywhere. Unless maybe the course was designed by a mountain biker and then you have bad course design.

KingCat

In my limited racing experience (15 races, 10 on CX bike 5 on MTB) it depends on the course. There is a course in Los Angeles area that is very hilly, no grass. I tested myself on both bikes and I was definitely faster on my full suspension mtb.
All other types of courses I seemed to be faster on the CX bike.
I found that the full suspension mtb bikes are just so much smoother that you don't get beat up like a CX bike.